Our immediate goal is a detailed analysis of the radiation response of two established in vivo lines (Slow and S102F) of the C3H mouse mammary tumor. This analysis will be especially concerned with evaluating the radiation response as it relates to the various growth kinetic parameters in the belief that such an approach will suggest a simplified model for optimizing protocols for fractionated radiation therapy. The initial studies are concerned with determining the single-dose TCD50/120s (i.e. the dose that shows local control in 50 percent of the tumors at 120 days post irradiation) for 250kVp x-rays when the tumors are under: 1) Normal physiological conditions; the mice are breathing air., 2) Anoxic conditions; a clamp is placed across the base of the tumor for 8 minutes before and then maintained in place during irradiation., 3) Hyperbaric conditions; the mice are breathing 95 percent O2 at 3 atmospheres of pressure. Preliminary data for part 1 suggest that the TCD50 for the Slow line may be about 8,000 rads while that for the S102F line is about 6,500 rads. The rate of DNA synthesis following 1500 rads of 250KVp x-ray is also being studied in both lines as a prelude to measuring cell loss following irradiation. The tumors are irradiated, then at selected times the mice are injected with 20 micron Ci 3H-TdR and killed one hour later. The tumor DNA is extracted and the radioactivity determined in the DNA and acid soluble fraction. The preliminary data show that the acid soluble radioactivity, in either set of tumors, does not vary as a function of time post irradiation. In the Slow line, the DNA radioactivity drops to a minimum (9 percent of control) at 16 hours. post-irradiation and then gradually increases with time to reach a level of 40 percent of control by 45 hours post irradiation. In the S102F line, the radioactivity drops to a minimum (10 percent) by 8 hours, stays at this level until 21 hours then rises to 70 percent of control at 24 hours and subsequently drops again to 15 percent at 46 hours post irradiation. These data suggest partial cell synchronization.